The government is recommending increased support for farm-scale anaerobic digestion (AD) at the expense of “solar farms" over 50kW, in an effort to maximise the benefit of limited resources.

The new consultation follows the launch in February of the fast-track review into how the Feed-in Tariffs (FITs) work for solar photovoltaic (PV) over 50 kW. This followed evidence of 169 MW of large scale solar capacity in the planning system - equivalent to funding solar modules on the roofs of around 50,000 homes if tariffs are left unchanged.

The government feels that leaving this unchanged would soak up most of the subsidy that would otherwise go to smaller schemes or other technologies. Such a development was not envisaged at the start of the programme.

The consultation also recommends increasing support for farm-scale AD, as it has received disappointing uptake so far. The heat component of AD is also supported through the Renewable Heat Incentive (RHI). This means that where the biogas is burnt to produce heat and power AD is eligible both for the RHI and FITs.

It could be argued that it doesn't matter where the PV modules are as long as they are generating electricity. But the government's concern is that PV be available to ordinary people and not big business.

Greg Barker, climate change minister, said: “I want to make sure that we capture the benefits of fast falling costs in solar technology to allow even more homes to benefit from feed in tariffs, rather than see that money go in bumper profits to a small number of big investors.

“These proposals aim to rebalance the scheme and put a stop to the threat of larger-scale solar soaking up the cash. The FITs scheme was never designed to be a profit generator for big business and financiers."

The consultation quotes figures saying that already prices for PV technology are 30% lower than originally projected. It argues that this means that the technology requires reduced support. Therefore installations larger than 50 kW will receive support as follows:

• 19p/kWh for 50kW to 150kW

• 15p/kWh for 150kW to 250kW

• 8.5p/kWh for 250kW to 5MW and stand-alone installations.

These compare with the tariffs that would otherwise apply from 1 April of:

• 32.9p/kWh for 10kw to 100kw

• 30.7/kWh for 100kw to 5MW and stand-alone installations.

These reductions are comparable to those in schemes in Germany, France and Spain, where tariffs for PV have been reduced sharply over the past year.

The new increased tariffs for AD, designed to make them more attractive, are:

• 14p/kWh for installations up to 250kW

• 13p/kWh for installations from 250kW to 500kW.

These compare with the tariffs that would otherwise apply from 1 April of 12.1p/kWh for AD up to 500kW. The tariff level set for biomethane injection into the gas grid under the RHI and also for small scale - below 200 kilowatt thermal (kWth) – combustion of the biogas produced by AD is 6.5 pence per kilowatt-hour of heat generated.

The idea is specifically to increase the energy obtained from waste through anaerobic digestion, not to promote energy crops, particularly where these might be grown instead of food crops. DECC is in discussions with Defra and others about ways to ensure this does not happen.

Subject to the outcome of the consultation and parliamentary scrutiny, the revised tariffs would be introduced from 1 August 2011.

Over 27,000 installations have been registered for the FIT scheme to date.

What is anaerobic digestion?

A survey last December found that 80% of farmers in the UK wanted to have solar photovoltaics on their roofs within the next three years - and yet the fact is, that in terms of the carbon saving and other benefits anaerobic digestion (AD) provides better value for money than solar PV.

For example, farmer Clive Pugh at Bank Farm, Mellington, near Churchstoke, Wales, put in his first AD plant 20 years ago. He now has a state-of-the-art, three chamber unit that provides all of the farm's own energy needs, and that for two homes and the farm dairy, as well as generating an income of up to £10,000 a month from supplying the National Grid - without the new FITs subsidy, because he was an 'early adopter' and so the scheme is excluded from it.

“We initially went for an anaerobic set-up because we needed a new slurry store and it was something we had been looking into for some years,” said Mr Pugh.

“It revolves around using the slurry from our 140-cow dairy herd. In order to keep the gas production fairly constant throughout the year, we also use poultry manure, silage effluent, waste silage, discarded milk and whatever other green waste we can get hold of.”

While ten cows are needed to produce 1kw of energy, in fertiliser value terms 1,000 gallons of separated liquid will provide around 30 units of nitrogen, 40 units of potash and 12 units of phosphate.

“The quality of our grass is certainly most noticeable these days, and our need for phosphate and potash is now nil. We also only need top-up units of nitrogen depending on the type of crops being grown,” Mr Pugh added.

In a typical plant, vats ferment farm slurry and crop waste (and can also process food waste) in the absence of oxygen to produce methane which can be used to generate heat and power.

The facility would normally be owned and operated by the farmer/farm business, but might sometimes be part of a co-operative venture. They often would not be approved to accept animal by-products at this scale.

The biogas produced in AD is a mixture of methane (65%) and carbon dioxide (35%) which can be used to generate heat through a boiler, or heat and power through a combined heat and power (CHP) system. In addition, following further processing, biogas is also a suitable fuel source for vehicles.

Hot water may be used on site, for example to heat polytunnels or greenhouses for market gardening. Some farms use AD to power a generator for the digester and pasteurisation. Other benefits include:

• it avoids landfilling of organic wastes;

• the biogas can be burnt as a fuel;

• there is a reduction in the use of fossil fuels, offsetting carbon dioxide emissions;

• it is a predictable and reliable source of electricity and energy, unlike wind power and PV;

The energy generating potential is determined by the size of the digester and waste feedstock composition.

A typical farm installation might be up to 0.5MW. A small farm using farm waste can produce enough heat to warm the digester and meet domestic heating requirements. If electricity is generated through CHP of 10kWe capacity, enough electrical energy could be generated to supply up to 13 homes.

A brand new installation can cost anything from £150,000 for a fairly basic liquid-only unit to more than £375,000 for an all-embracing 120kW producing version.

This high initial cost is why the technology needs support at this stage. Without support, simple economic payback is approximately 20 years. Factoring in savings made in waste disposal, according to the Carbon Trust, mean that payback times for installations tend to be under five years. Compare this to solar PV in much of the UK, which is two to three times longer.

A range of AD scales exists, from single on-farm digesters through to large centralised anaerobic digesters (CAD) collecting waste from a larger surrounding area.

These CADs will usually accept animal by-product wastes for digestion. The gas produced at this scale can also be used for other purposes, for example to power vehicles or be injected into the National Grid.

AD at this scale is economically viable and requires little support. Most plants operate as co-digestion plants with slurries, in additional to wastes from the food, brewing and other industries.